HKUST ELEC 4620 Photonics and Optical Communications - presentation

1. Free space optical communication using
orbital angular momentum multiplexing
Emma Yann Zhang
Hong Kong University of Science and Technology
May 11, 2013

2. History of Orbital Angular Momentum
1932 – C. Darwin identified another part of angular momentum
in photons besides spin.
• Angular momentum of photons has external part due to motion of the
center of gravity and intrinsic part due to spin.
During high order atomic transitions,
photon emits at a distance r from atom. The
product of linear momentum of photon and
radial distance contributes to an angular
momentum independent of spin.
C.G. Darwin. (1932). “Notes on the Theory of Radiation,” Proceedings of the Royal Society of London.

3. History of Orbital Angular Momentum
1992 – L. Allen found all helically phased beams have OAM,
and can be generated using cylindrical lenses
• OAM is a natural property in helically phased beams
• LG modes (helical beams) can be easily produced using cylindrical lenses
mode converters from HG modes.
L. Allen et al. (1992). “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser
modes,” Physical Review A
Helical light beams with OAM = +ħ Cylindrical lenses mode converter

4. History of Orbital Angular Momentum
1992 – Helical beams with optical vortex generated using forked
diffraction grating.
• More lines at the bottom, light diffracts at different angle
• Changing the number of split lines changes the mode produced.
V. Yu. Bazhenov et al. 1992."Screw dislocations in light wavefronts." Journal of Modern Optics 39.5
Forked diffraction grating produces screwed wavefronts
with optical vortex at the center

5. History of Orbital Angular Momentum
1995 – OAM can be transferred and rotate microscopic particles
• Direction of rotation
determined by the sign of the
OAM state.
• A laser beam of a few mW
gives a rotation speed of 1-10
Hz.
He, H., et al. 1995. "Direct observation of transfer of angular momentum to absorptive particles from a laser beam
with a phase singularity." Physical Review Letters, 75.5
The rotation of a trapped red blood
cell under (i)15ħ, (ii)−15ħ.
Dasgupta et al. 2011.Optics Express, Vol. 19, Issue 8

6. History of Orbital Angular Momentum
2004 – Gibson et al. demonstrated free-space optical
communication with OAM-carrying light beams
• Information encoded in OAM states,
transmitted in air over 15m.
• Computer generated hologram on
spatial light modulators (SLM)
Gibson, Graham, et al. 2004. "Free-space information transfer using light beams carrying orbital angular
momentum." Opt. Express 12.22

7. History of Orbital Angular Momentum
2012 – Terabit data transfer rate using OAM multiplexing.
• Data rate of 2.56Tbit/s over 1
meter in free space
• Combining OAM multiplexing
and polarization multiplexing to
increase capacity and spectral
efficiency
Wang, Jian, et al. 2012."Terabit free-space data transmission employing orbital angular momentum
multiplexing." Nature Photonics.

8. Helically phased beams
 Azimuthal phase dependence
 Optical vortex
 Carry OAM of per photon
l = 0
l = -1
l = +1
l = +2
l = +3
http://www.gla.ac.uk/schools/physics/research/groups/optics/research/orbitalangularmomentum/

9. Visualize helical waves…

10. Laguerre-Gaussian Mode
 LGpl
 p – number of radial nodes
 l – azimuthal index

11. Orbital and spin angular momentum
Angular momentum Orbital Spin
Wave fronts: Helically phased waves Circularly polarized waves
AM per photon l ħ σħ
No. of states ±1, 2, 3, 4….. ±1
Each photon has unlimited OAM states that can carry information without interference!

12. OAM Multiplexing in FSO Communication System
Laser resonator
Generating LG
beams
Multiplexing
LG beams
Propagating in
free space
Demultiplexing
LG beams
Recover data
Holograms
using SLM

13. Methods of Generating Light Carrying OAM
 Asymmetric laser resonators (direct generation)
 Cylindrical lenses
 Forked diffraction grating
 Spiral phase plate
 Computer generated holograms
iPad app: iHologramSpatial Light Modulator (SLM)

14. Advantages of OAM Multiplexing
Advantages Limitations
 High data capacity
 Higher when combined
with other multiplexing
methods
 Enhanced security
 Capacity limited by
atmospheric turbulence
 Unable to use in long haul
fiber communication systems
 Mode coupling in waveguide
 Wong, KLGG, et al. 2012. "Quantization of
orbital angular momentum in helically twisted
photonic crystal fiber." Frontiers in Optics.

15. Applications
 Deep space and near-Earth
optical communications
 LAN to LAN connections
in campus / city area
network
Djordjevic, I. B. (2011). Deep-space and near-Earth optical communications by coded orbital angular momentum
(OAM) modulation. Opt. Express, 19

16. Interesting videos on OAM
 Talk by Miles Padgett at SPIE Photonic West: Miles J.
Padgett Photonics West 2013 plenary talk: Light in a
Twist: Optical Angular Momentum
http://www.youtube.com/watch?v=5yyTEUYY8o8
 Short introductory on OAM and optical tweezers:
Miles Padgett: Optical tweezers and twisted beams of
light http://www.youtube.com/watch?v=2hdKXMRKSY8